Piezoelectric ceramic transducer



Oct. 21, 1969 N. RUDNICK PIEZOELECTRIC CERAMIC TRANSDUCER 2 Sheets-Sheet1 Oct. 21, 1969 lN. RuDNlcK 3,474,268

PIEZOELECTRIC CERAMIC TRANSDUCER Filed April 21, 1966 2 Sheets-Sheet 2United States Patent O 3,474,268 PIEZOELECTRIC CERAMIC TRANSDUCER NormanRudnick, Piscataway, NJ., assignor to Gulton ndustries, Inc., Metuchen,NJ., a corporation of New ersey Filed Apr. 21, 1966, Ser. No. 544,242Int. Cl. H01v 7/00 U.S. Cl. S- 8.5 8 Claims ABSTRACT OF THE DISCLOSURE Apiezoelectric transducer having a homogeneous monolithic body having oneelectrode embedded in the center of the body and the other electrodecompletely surrounding the body except at one end thereof. Themonolithic body operates in the shear mode.

The present invention relates to bar or strip type piezoelectric ceramictransducers which are to produce a mechanical 4movement in response tothe application of a voltage thereto, and has its most important (butnot its only) application to transducers of this type which are toimpart appreciable thrust or pulling forces at high rates as deteminedby voltage pulses applied at relatively high random pulse repetitionrates.

The production of a bar or strip type piezoelectric transd-ucer whichmust operate at appreciable force and displacement magnitudes and athigh rates, such as hundreds or thousands of force pulsations persecond, and at reasonably low voltages generally requires a very thinsandwich of two or more piezoelectric elements connected in parallel.Frequently, such a piezoelectric transducer must have a thickness wellunder 100 mils (like 20-30 mils or less). Prior to the presentinvention, due to the nature of the transducer constructions, it wasexceedingly difiicult to manufacture such thin bar or strip typetransducer sandwiches having the required ruggedness and sensitivity.Thus, it was heretofore the practice to manufacture ceramic transducersof the type referred to by first fabricating individual two terminaltransducer elements and then cementing them together. The cementing ofthe transducer elements was disadvantageous because, among otherreasons, it was difiicult to handle the thin transducer elements duringthe cementing process without breaking the same. Also, the ceramictransducer elements are formed by firing unusually thin strips of rawceramic which easily warp during the firing process. Additionally, theadhesive used to cement the transducer element together addedappreciably to the thickness of the resulting sandwich to reduce thedisplacement per unit thickness achieved with a given electric field.Moreover, the electrode configurations used created arcing, terminalsoldering and contact problems, and the most effective use of thepiezoelectric material was not achieved.

It is, accordingly, one of the objects of the invention to provide avery thin sensitive rugged bar or strip type ceramic piezoelectrictransducer sandwich in which appreciable intermittent movement can beimparted thereto at high random rates by voltage pulses fed thereto athigh pulse repetition rates and relatively modest voltage levels.

Another object of the invention is to provide a bar or strip typeceramic piezoelectric transducer sandwich which produces a maximumdeflection for a given voltage and for a given thickness and volume ofpiezoelectric material utilized.

Still another object of the invention is to provide a ceramicpiezoelectric transducer sandwich as described which is so thin thatlarge voltage gradients are present ice and yet where arcing between theelectrodes does not readily take place.

A further object of the invention is to provide a ceramic transducersandwich as described which, because of its mode of construction, can beeasily manufactured in extremely thin cross-sections without warping andother problems and by mass production techniques.

A still further object of the invention is to provide a thin ceramictransducer sandwich as described to which strong solder connections orother good contact connections can be readily made.

Another object of the invention is to provide a ceramic transducersandwich as described where the movement of the transducer unit is alongitudinal expansion or contraction thereof in response to a signalvoltage applied across the thickness thereof.

To best appreciate the significance of the present invention, it shouldbe understood that piezoelectric ceramic transducers generally producevery small mechanical displacements when electrically stressed unlessoperated under resonant conditions or in bending or flexural modes. Theoperation of piezoelectric transducers under resonant conditions hasserious shortcomings. For example, resonant operation may not besuitable because it may be difficult to design a transducer to operateat the desired frequency or because the transducer must be operated atrandom times. Also, with resonant operation of transducers small forcesopposing the motion thereof will suppress much of the amplified motionresulting from the resonant mode of operation thereof. Bending oriiexural modes of operation of a piezoelectric transducer unit achievelarge displacements only at the sacrifice of the ability to apply strongthrust or pulling forces.

The best electromechanical coupling in piezoelectric transducer units isattained when the displacements thereof are in line with the appliedelectric fields. In this case, however, the applied voltage mustgenerally be very large to produce relatively large movements. In thecase where the displacements involved are transverse to the appliedelectric field, relatively large movement accompanied by relativelylarge forces can be generated without requiring very large voltages. Thetransducer of the present invention is constructed to operate in thelatter mode.

In the most advantageous form of the invention, two or more pairs 4ofthin elongated strips of piezoelectric ceramic material are securedtogether with one or more electrode layers between each pair of strips.The electrode layer between the strips of each pair of strips ofpiezoelectric material is spaced appreciably from all margins of thesandwich except one which is most desirably one end margin of thepiezoelectric sandwich. There is located between the contiguous pair ofstrips of piezoelectric material (where more than one pair is utilized)an electrode layer spaced appreciably from all margins of the sandwichexcept one which is most desirably at the opposite end of thepiezoelectric sandwich from said one end. The electrode layers encompassmost of the cross sectional area of the transducer unit in a planeparallel to the opposite faces (i.e. transverse to the thickness) of thetransducer unit. A first region of conductive material is applied to anend face of the piezoelectric sandwich to which the electrode layersbetween the strips of each pair of strips extend. This region ofconductive material preferably extends a short distance around the sidesof the piezoelectric sandwich materially to increase the area ofconductive material occupied by the first region of said material whichforms a terminal for the transducer unit of appreciable area, thoughoccupying only an insignificant percentage of the area of the entiretransducer unit. A second region of conductive material is preferablyapplied to the remaining surface area of the piezoelectric sandwichexcept for a narrow insulating gap separating the same from said lirstregion of conductive material. The second region of conductive materialwill thus make electrical contact with any inner electrode layersextending to said other end margin of the piezoelectric sandwich. Thesecond region of conductive material forms a terminal for the transducerunit and with any other inner electrode connected thereto, electrodemeans which cooperates with the inner electrode layers extending to thefirst region of'conductive material to elfect the desired displacementwhen a voltage is applied to the transducer unit.

A feature of substantial importance is that the piezoelectric sandwichis constructed so that the piezoelectric material throughout thesandwich forms a single, continuous, homogeneous monolithic body of suchmaterial extending between said regions of conductive material andaround the edges of all of the inner electrodes except the edges thereofextending to the margins of the piezoelectric sandwich. The monolithiccharacter of the piezoelectric sandwic'h can be achieved bysuperimposing thin sheets `of raw ceramic material held together with abinder and having the various aforesaid electrode layers silk screenedor otherwise coated on the surfaces of the layers of ceramic material tobe located within the sandwich, applying pressure to the raw ceramicsandwich and then firing the same to sinter the various sheets of rawceramic material into a solid monolithic structure. The transducer unitdescribed is formed into a piezoelectric unit by simply polarizing thepiezoelectric ceramic mate- `rial by the application of a polarizingvoltage between the terminals of the transducer unit.

For a single, long, thin element of piezoelectric ceramic electrode onits two major faces and poled in the direction of its thickness, therelation between longitudinal displacement and applied voltage is:

Where AL is the displacement in meters from the -unstrained length L, Vis applied voltage in volts, d3, is the transverse piezoelectric straincoeiicient with units of meters/meter per volt/meter, and t is thethickness of the transducer in meters. It can be seen that thedisplacement can be increased without change in voltage by increasingthe length and `decreasing the thickness.

The transducer unit described above comprising two or more strips orlayers of piezoelectric material secured together in a manner to form acontinuous, homogeneous, monolithic piezoelectric body constitutes amultiplicity of piezoelectric elements electrically connected inparallel. Such a transducer unit can deliver the same displacement witha greater thrust at the same voltage than a single layer transducerunit. Also, in the monolithic multi-layered structure described, theapplied voltages affect substantially all portions of the piezoelectricbody for maximum response and arcing is prevented because the closelyspaced edges of the electrodes are separated by a high di-electricceramic material. Moreover, it is much stronger in its raw unfired state(as well as its completed state) than a single layered structure, sowarping cannot readily occur during firing thereof, and it affords suchmounting advantages as a grounded outer surface where needed and greaterresistance to breaking where cementing or clamping of the transducerunit is required. Also, strong solder or other good electricalconnections can readily be made thereto.

The transducer unit of the invention described above can be mountedwithin a holder which confines the transducer on all sides except oneend thereof. Then, when the voltage is applied to the terminals of thetransducer unit, all of the displacement will occur at one end of thetransducer element. The holder can include terminal strips which contactthe aforesaid first and second regions of conductive material and leadto screw or other ter- 4 minals for connecting the ends of conductorwires. Alternatively, wires can be directly soldered to the regions ofconductive material referred to.

The amount of motion producible by the transducer unit yof the inventionwith presently available piezoelectric ceramic compositions makes ituseful wherever small displacements are required `with a minimum ofmachinery and electrical input. Such applications lie in the areas ofoptical adjustments, switching. actions, pumping devices, and, inparticular, rapid printing mechanisms. For example, a 4.6 inch long,two-layered strip, 0.20 inch wide and 0.018 inch thick, has a lowestresonant frequency of over 10,000 cycles per second. For application ina printing mechanism, the voltage pulses fed thereto,which are desirablyfed at a pulse repetition rate well below the lowest resonant frequencythereof, can be ata 4rate of several hundreds and even thousands ofpulses per second, which can provide repeated random thrusts ata ratefar exceeding the rates attainable by conventional printing mechanisms.Whereas conventional printing mechanisms operate at -rates -of the orderof 3 to 4 lines per second, instruments using the piezoelectrictransducer of the present invention can be operated to produce to 200lines per second and higher.

Fora clearer understanding of the present invention, reference should bemade tothe drawings wherein:

FIG. 1 is a perspective view of a rapid printing mechanism having aprinting head with the transducer unit of the present inventionincorporated therein;

FIG. 2 is a greatly enlarged, fragmentary sectional view through theprinting head of FIG. 1, taken along section line 2--2 thereof;

FIG. 3 is another sectional view of the printing head of FIGS. 1 and 2,taken along section lineI 3 3 in FIG. 2;

FIG. 4 is a perspective view of a transducer unit used in the printinghead of FIGS. 1 through 3, and having one internal electrode;

FIG. 5 is an enlarged horizontal longitudinal sectional view through thetransducer unit of FIG. 4, taken along section line 5-5 therein;

FIG. 6 is a greatly enlarged fragmentary, transverse sectional,perspective view of the transducer unit of FIGS. 4 and 5;

FIG. 7 is a diagrammatic view of the transducer unit of FIG. 4 connectedinto an electrical circuit to operate the same;

FIG. 8 is a curve showing exemplary bias and signal voltages applied toa transducer unit of the invention;

FIGS. 9 through 13, are a series of views illustrating a preferredmethod of mass producing transducer units like that illustrated in FIGS.4 through 6; and

FIG. 14 is a longitudinal vertical sectional view through a form oftransducer unit of the present invention which utilizes a number ofinternal electrodes.

As previously indicated, one of the most important applications of thepresent invention is in a high speed printing mechanism where thetransducer elements are operated at very high speeds. FIG. 1 illustratessuch a high speed printing mechanism which includes a printing headgenerally indicated by reference numeral 1 which carries on the innerface thereof a row of thin transducer units 2 which are mounted inclosely spaced relation with their thin dimensions extending in thedirection of the spacing thereof in individual cavities 5 in a housing`6 forming part of lthe printing head. The vindividual transducer units2 are most advantageously thin and elongated as shown in FIG. 4 and ofrectangular cross-section. Each transducer unit is closely confined butnot constricted against longitudinal sliding movement on three sidesthereof, as shown in FIGS. 2 and 3, and have an inner end projectinghorizontally from the printing head housing 6 -where it is free to movelongitudinally outwardly. A small piece of metal or other similarmaterial 7 is preferably secured by an adhesive or otherwise on theprojecting end face or edge of each transducer unit 2. The metal facedend of each transducer unit may be positioned to engage a printingelement or, as illustrated, one side of a moving strip of paper whoseother side passes by a stationary or moving raised indicia carryingmember 12. As a transducer unit 2 is thrust outwardly momentarily, animprint is made on the point of paper 10 engaged by (or opposite the)small end face of the transducer unit involved. The metal facing 7 oneach transducer unit protects the end thereof from wear.

4Each transducer unit has a pair of terminals formed by insulatedregions of conductive material 8 and 9, as best shown in FIG. 4. Theconductive region 8 preferably forms a narrow end cap around one end ofthe transducer unit and so covers the end face and a narrow strip on allside faces and edges of the transducer unit occupying only aninsignificant fraction of the length of the transducer unit. The otherregion 9 of conductive material is spaced from the narrow region l8 ofconductive material by a narrow insulating gap 10, and preferablyoccupies, except for the gap 10i, and the area encompassed by theconductive end cap 8, all remaining surface areas of the transducerunit, which is most of the surface area thereof. The regions 81 and 9 ofconductive material form terminals for the transducer unit, the latterregion also forming an outer displacement imparting electrode. Theportions of the large region 9 of conductive material covering theopposite faces 11 and 13 of the transducer unit confronts the oppositefaces of at least one inner electrode 15 imbedded Within a body ofpiezoelectric material 17 constituting most of the volume of thetransducer unit 2. As best shown in FIGS. 5 and 6, the inner electrode15 is in a plane parallel to the opposite faces 11 and 13 of thepiezoelectric body. It occupies most of the crosssectional area of thepiezoelectric body in said plane and is spaced from all margins of thepiezoelectric body 17 except the end thereof including the narrow regionor lcap 8 of conductive material with which it makes electrical contactby extending to the end face 19 thereof (FIG. 5).

The transducer unit 2 is pre-polarized in a direction transverse to thethickness thereof as shown by arrows 21 in FIG. 6 by application of asuitable dire-ct current voltage between the conductive regions orterminals 8 and 9 thereof. As previously indicated, the piezoelectricbody 17 forms a single, continuous, homogeneous monolithic body ofpiezoelectric material, so that the polarization referred to makes theentire volume of the piezoelectric body active in the piezoelectricprocess. When a signal voltage is applied across the terminals 8 and 9in a direction to oppose this polarization, the transducer unit willcontract in thickness which, in turn, results in an expansion of thelength of the piezoelectric body, which is the desired movement in thehigh speed printing mechanism application of the invention illustratedin FIG. 1. If the aforesaid signal voltage reinforces the polarization,the transducer unit would expand in thickness and contractlongitudinally. l

Where a longitudinal expansion of the transducer unit 2 is desired, abiasing action voltage 23 (FIG. 7) is desirably applied in a directionof the original poling orientation, and a source of signal voltage 25 isconnected in series with the biasing voltage source 23 between thetransducer terminals 8 and 9, so that the signal voltage is superimposedon and opposes the biasing voltage, as illustrated Iby the curve of FIG.8. When the signal voltage is removed, it is apparent that the biasvoltage source 23 will re-establish the original polarization, if anydepoling action results from the signal voltage. If desired, because ofthe bias voltage, the signal voltage may be raised substantially beyondthe limits di-ctated by an unbiased transducer unit without de-polingthe transducer or reversing its direction of polarization, therebyincreasing the amount of elongation that can be produced.

In the printing head application of the invention shown in FIGS. 1through 3, electrical connection is made to the terminals 8 and 9 ofeach transducer unit in the housing 6 in any suitable Way, such as byplacing within the housing 6, which is shown as being made of a moldedinsulating plastic material, a pair of longitudinally spaced metal rings25 and 27 which line each cavity 5 :and are respectively engaged b`y theinsulated terminals 8 and 9 formed by the aforementioned regions ofconductive material on the piezoelectric body of the associatedtransducer unit as best shown in FIG. 3. The rings 25 and 27 areconnected by conductor strips 25 and 27 to the rear face of the housing6 where they are engaged by suitable terminal posts or screws 31 and 33,respectively, to which the bared end of signal wires 35 and 37 andrespectively connected. Each pair of signal wires 3S and 37 extend tothe circuit including the biasing voltage source 23 and the signalvoltage source 25, as shown in FIG. 7.

The transducer units 2 can be made in a wide variety of sizes. For oneprinting head application, the piezoelectric body had an over-allthickness of between 18 and 20 mils, a length of about 41/2 inches, anda width of about .2 inch. With such a construction, with a bias ofnegative 200 volts, an elongation of over 1 mil resulted for a signalvoltage of plus 400 volts. This produced a voltage gradient of 20 voltsper mil and yet no arcing occurred. This voltage gradient could probablybe raised to 30 volts per mil and higher without arcing problems. Themonolithic character of the piezoelectric material forming the body 7added materially to the strength, effectiveness and compactness of thetransducer unit 1.

Refer now to FIGS. 9 through 12 which illustrate the manner in which thetransducer units 2 are preferably made. As shown in FIG. 9, thinflexible at sheets 17a- 17b of raw piezoelectric ceramic material lareprovided which are sized to form a number of transducer units. Theformulation of the slip used contains appropriate proportion of binderand vehicle along with the ceramic powders needed to produce apiezoelectric body, such as one of the family of barium titanates orlead titanate-zirconates. An exemplary formula for the ceramic is amodified lead titan-ate zirconate Pb98 5La1.5(Ti46Zr54)03. Since thesheets are enforced by binder content, they may be made very thinwithout major problems in handling. The thickness of the sheets 17a-17bdepends upon the desired thickness of the piezoelectric bodies desired,and consideration must be given to substantial shrinkage factors commonin the firing of piezoelectric ceramic materials (shrinkage factors of25% are not uncommon).

The surface of one of the sheets 17a which is to confront the othersheet 17b has applied thereon metallic coatings 15a as shown in FIG. 9which all extend to the same edge 17a' of the sheet 17a. The materialwhich is coated on the sheet 17a may be a suspension of ne noble 4metalpowder, such as platinum, palladium, platinum-gold mixture, or similarmetal which will remain inert and survive subsequent firing at hightemperatures. The sheet 17b is then placed on the coated side of thesheet 17a (FIG. l0) and the sheets 4are pressed to promote intimacy ofcontact and eliminate entrapped air from between the sheets. A printingpattern comprising lines 40 may be suitably screened or otherwise formedon the outside of the sheet 17b best shown in FIG. 9 to indicate wherethe laminate body should be cut to form separate individual transducerunits The individual elements are then red at a suitable temperature,as, for example, in a kiln with a controlled atmosphere into which leadoxide vapors are introduced Initially, the transducer elements areheated slowly to drive off organic binders and the like. The units arethen heated to a much higher temperature, for example, a temperature of2350 degrees Fahrenheit to sinter the ceramic material and solidify thesame to form a solid monolithic block. The internal metal electrodes 15become a continuous, conducting film imbedded within the ceramicmaterial and emerging only -at one end to provide access for externalelectrode connections as indicated previously.

i. After the firing operation, a strip of adhesive material l42 (FIG.11) Vis wound around each piezoelectric element intimate contact withthe piezoelectric material. This .forms a strong bond between the silverand the ceramic. The tail 42 of the adhesivey strip 42 is grasped andunraveled, which leaves the aforementioned insulating gap 10. Theresulting transducer unit comprises two electroded layers ofmonolithically bonded piezoelectric material which are electricallyconnected in parallel, as shown in FIG. 7.

As previously indicated, if a greater thrust or pulling force isdesired, additional pairsof electroded piezoelectric layers are added tothe transducer laminate during its fabrication. In such case, in orderthat the piezoelectric material of the body 17 be most effectively used,it is necessary thatA both sides of each internal electrode extending tothe narrow region of conductive material 9 confront an electrodeextending to the large region of conductive material 9. Assuming thatthe second region of conductive material 9 covers both faces 11 and 13of the piezoelectric body as in the preferred form of the invention,there would be an odd number of internal electrodes imbedded within thebody of piezoelectric material with an even number (n) of electrodesextending to the narrow region of conductive material 8 and (la-1)internal electrodes extending to the opposite ends of the piezoelectricbody where they contact the large region of conductive material 9. Eachof the latter electrodes is positioned between and spaced from a pair ofthe (n) electrodes extending to the region of conductive material 8.FIG. 14 illustrates a transducer unit 2' with an outer piezoelectricbody 17 having three internal electrodes, the outer electrodes 15-15extending to the narrow region of conductive material 8 and immediatelyconfronting the large region of conductive material 9' for most of thelength and width of the body 17. Another internal electrode 45 -ispositioned in spaced relation between the electrodes 15-15' and extendsto the end face of the piezoelectric body 17 opposite to that includingthe narrow region of conductive material 8.

It should be understood that numerous modifications may be made in themost preferred forms of the invention described above without deviatingfrom the broader .aspects of the invention.

I claim:

1. A piezoelectric transducer unit comprising: a thin plate-like body ofpiezoelectric ceramic material having opposite ends to which movement isto be imparted in a direction along a line extending between the endsthereof by application of a voltage acrossthe thickness thereof, saidbody of piezoelectric material having imbedded therein at least oneinner electrode which is spaced from all margins of the body except oneend edge thereof to which the electrode extends, said electrodeencompassing most of the cross sectional area of the body in a planetransverse to the thickness of the body, a first region of conductivematerial at least on the outside edge of said piezoelectric body at saidone end thereof which region of conductive material makes electricalcontact with said inner electrode extending to the end edge, a secondregion of conductive material on the outside of said body and spaced byan insulating gap from said first region of conductive material, saidfirst region of conductive material constituting one terminal of thetransducer unit and said second region of conductive materialconstituting another terminal and an electrode of the transducer unitwhich immediately confronts, is parallel to and is spaced from at leastone face of said inner electrode over an extensive area thereof, saidbody of piezoelectric material being polarized in the direction of thethickness thereof, wherein application of a signal voltage across saidterminals will effect contraction or expansion of the piezoelectric bodytransversely thereof resulting respectively in the expansion orcontraction of the ends thereof, and said body of piezoelectric materialbeing a single, continuous, homogeneous monolithic body of such materialextending between `all of said electrodes and around all the edges ofeach inner electrode except the one edge extending to said one end ofthe piezoelectric body.

2. The piezoelectric transducer unit of claim 1 wherein said firstregion of conductive material is located entirely in a small region atsaid one end of the piezoelectric body and said second region ofconductive material extends for most of the length of the piezoelectricIbody.

3. The transducer unit of claim 2 wherein there s an odd number of innerelectrodes in said body of piezoelectricmaterial, said second region ofconductive material covers substantially all the surface areas of thepiezoelectric body on all sides thereof, and the portions of said secondregion of conductive material on the opposite faces of the piezoelectricbody each confronts a corresponding portion of an inner electrode whichis electrically connected to said first region of conductive material atsaid one end of the piezoelectric body.

4. The transducer unit of claim 3 wherein said rst region of conductivematerial extends around the side o the piezoelectric body for a shortdistance representing an insignificant portion of the length of thebody, to provide a much larger area of conductive material than the areaof the end edge thereof.

5. The transducer unit of claim 2 wher-ein there are n parallel spacedinner electrodes like said one electrode within said piezoelectric bodywhich extend to said one end thereof to make contact with said firstregion of conductive material, n being an even number, and there are n-lelectrodes in spaced parallel relation to and interleaved with said nparallel electrodes, each of said n-l electrodes extending to a surfaceof the piezoelectric body which is covered by said second region ofconductive material to make electrical Contact therewith, the portionsof said second region of conductive material on said opposite faces ofsaid piezoelectric body immediately confronting the outermost of said nelectrodes.

6. The transducer unit of claim 5 wherein the surface of thepiezoelectric body to which said n-l inner electrodes extend is the endedge thereof opposite the end edge to which said n inner electrodesextend, and said second region of conductive material covering theformer end edge in addition to the opposite faces of the piezoelectricbody.

7. The transducer unit of claim 1 wherein the piezoelectric body is anelongated body whose width is only a small fraction of the lengththereof and whose thickness is only a small fraction of the widththereof.

8. The transducer unit of claim 7 wherein the surface of thepiezoelectric body to which said n-l inner electrodes extend is the endedge thereof opposite the end edge to which said n inner electrodesextend, and said second region of conductive material covering theformer end edge in addition to the opposite faces of the piezoelectricbody.

References Cited UNITED STATES PATENTS 1,860,529 5/1932 Cady 310-8.62,269,403 1/ 1942 Williams S10-8.6 2,451,966 10/ 1948 Massa 310-863,115,588 12/1963 Hueter S10-8.6 3,258,617 6/1966 Hart B10-8.6 3,378,7044/ 1968 Miller 310-85 I. D. MILLER, Primary Examiner I Us. C1. XR.

